Redirecting System.out.println() output to a file in Java

System.out.println()  is used mostly to print messages to the console. However very few of us are actually aware of  its working mechanism.

  • System is a class defined in the java.lang package.
  • out is an instance of PrintStream  , which is a public and static member of the class System.
  • As all instances of PrintStream class have a public method println(), hence we can invoke the same on out as well. We can assume System.out  represents the standard Output Stream .

One interesting fact related to the above topic is, we can use System.out.println() to print messages to other sources too (and not just console) . However before doing so , we must reassign the standard output by using the following method of System class:

System.setOut(PrintStream p);

PrintStream can be used for character output to a text file. Below program creates the file A.txt and writes to the file using System.out.println(

// Java program to demonstrate redirection in System.out.println()
import java.io.*;

public class SystemFact
{
    public static void main(String arr[]) throws FileNotFoundException
    {
        // Creating a File object that represents the disk file.
        PrintStream o = new PrintStream(new File("A.txt"));

        // Store current System.out before assigning a new value
        PrintStream console = System.out;

        // Assign o to output stream
        System.setOut(o);
        System.out.println("This will be written to the text file");

        // Use stored value for output stream
        System.setOut(console);
        System.out.println("This will be written on the console!");
    }
}

In very similar fashion we can use System.out.println() to write to a Socket’s OutputStream as well.

This article is contributed by Ashutosh Kumar. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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